1. Field of the Invention
The present invention relates to a thick-film resistor paste suitable for forming a thick-film resistor and a thick-film resistor formed using a thick-film resistor paste.
2. Description of the Related Art
For example, a thick-film resistor paste is generally comprised of a glass composition, a conductive material, and an organic vehicle as main ingredients. The glass composition is included for adjusting the resistance value and having adhesion. This thick-film resistor paste is printed on a substrate, then fired to form a thick-film resistor of a thickness of 5 to 20 μm or so. Further, in this type of thick-film resistor paste (thick-film resistor), normally a lead-ruthenium oxide etc. is used as the conductive material, while a lead oxide (PbO)-based glass etc. is used as the glass composition.
However, in recent years, environmental issues have become hotly debated. For example, in solder material etc., elimination of lead has been sought. Thick-film resistor paste and thick-film resistors are no exception. Therefore, if considering the environment, use of a lead-ruthenium oxide as the conductive material and use of a PbO-based glass as the glass composition have to be avoided.
In view of this situation, lead-free thick-film resistor pastes and thick-film resistors are being researched in various sectors. For example, Japanese Patent Publication (A) No. 2003-197405 describes to introduce into the resistor paste for example CaTiO3 in an amount over 0 vol % and not more than 13 vol % or NiO in an amount over 0 vol % and not more than 12 vol %. Further, it describes that simultaneous addition of CuO, ZnO, MgO, and other additives is more preferable. Further, this publication describes that by doing this, it is possible to provide a lead-free resistor paste suitable for obtaining a resistor with a high resistance value, yet with a small temperature characteristic of the resistance value (TCR) and short time overload characteristic (STOL).
However, the technology described in Japanese Patent Publication (A) No. 2003-197405 does indeed improve the TCR and STOL, but also discloses samples with a TCR within ±100 ppm and with a STOL close to zero. However, sufficient values of both TCR and STOL are obtained with only extremely limited compositions. In the majority of compositions, the STOL is a value of 1% or more even if small.
If compositions giving sufficiently good values of both TCR and STOL are limited in this way, for example, the freedom of other characteristics is also limited and problems are liable to occur in the design of the resistor paste. Therefore, greater improvement is desired.